Igneous Petrology (Part 3 - Igneous Rocks Classification, Igneous Rocks Association) Flashcards
A.K.A mafic index, is the proportion of mafic minerals in the total population of felsic and mafic minerals
Color Index (CI)
A much more accurate method of obtaining a mode for any coarse-grained rock done by moving a thin section incrementally on a grid system such that at least 400 minerals are tabulated for each thin section.
Point count analysis
The first norm classification devised by Cross, Iddings, Pirsson and Washington
Cipw Norm
A classification by the IUGS based on the modal concentrations of Quartz, Alkali feldspars, Plagioclase feldspars, and Feldspathoids, which applies for igneous rocks with >10% felsic minerals and <90% mafic minerals (M) content by volume.
IUGS Classification
A quantitative analysis that determines the abundance of a component of a material.
Normalization
Steps in Normalization
- Identify included minerals
- Sum up the quantity
- Normalize components
- Plot the percentages
State what minerals are in the QAPF Mineralogy
Q- Quartz, tridymite, cristobalite, coestite, stishovite
A- Orthoclase, microline, sanidine, perthite, anortholase
P- Anorthite, Bytownite, Labradorite, Andesine, Oligoclase, Albite
F- Nepheline, sodalite, cancrinite, leucite, etc.
Suites of rocks that form in response to similar geologic conditions
Petrotectonic Associations
3 Petrotectonic Associations
- Divergent Plate Boundary Magmatism
- Convergent Plate Boundary Magmatism
- Intraplate Magmatism
Primarily driven by the decompression of the asthenosphere in response to lithospheric extension resulting in partial melting of mantle peridotite at divergent margins, producing basic (mafic) melts that rise to solidify and produce oceanic crust.
Divergent Plate Boundary Magmatism
A sequence of the ocean lithosphere containing four distinct layers
Ophiolitic Sequence
Slices of ocean lithosphere preserved in landmasses.
Ophiolites
Explain the 4 Layers
Layer 1: well stratified marine pelagic sediments
Layer 2a: containing pillow basalts
Layer 2b: containing diabase dikes
Layer 3: gabbro section and layered peridotite
Layer 4: Boundary of the mohorovic discontinuity
Basalts and gabbros that are geochemically distinct which formed from partial melting of lherzolite at ocean spreading centers, and contain plagioclase, augite, hypersthene, pigeonite, diopside and olivine.
Mid-Ocean Ridge Basalts (MORB)
Two types of MORB
Normal Morb (N-MORB)
Enriched Morb (E-MORB)
Strongly depleted in highly incompatible elements
N-MORB
Contains higher incompatible elements
E-MORB
What are the “incompatible elements”
Light rare earth elements (LREE)
Large ion lithophile (LIL)
High field strength elements (HFS)
- lons that are easily accommodated in the crystal lattice and mineral structure.
- Concentrated in the solids
Compatible Elements
- lons that do not fit into the mineral structure of rock- forming minerals at the time of crystallization of the magma, leading to accumulation in the residual magma and concentration in late-stage differentiation.
- Concentrated in the melt
Incompatible Elements
What is compatibility in geochemistry?
indicates the presence of certain trace elements in rocks
Generates chemically diverse igneous assemblages that erupt in the convergent margins widely distributed around the globe, occurs for thousands of kilometers parallel to the trench, and up to 500 km perpendicular to the trench in the direction of the subduction.
Convergent Plate Boundary Magmatism
Variables that can diversify magma composition
- Composition and geometry of crust
- Composition of material undergoing anatexis
- Flux melting
- Diversification process
- Dip angle of the subduction zone
The calc-alkaline association - signature volcanic rock suite of convergent margins which is one of the most voluminous rock assemblages on Earth.
Basalt, Andesite, Dacite, Rhyolite (BADR)
Tholeiites containing 16% wt. of Al2O3 concentrations
Arc tholeiites
Contains higher alkali concentration without iron enrichment
Calc-alkaline tholeiites
Found in youthful island arc systems. (>52-57% SiO2)
Basaltic andesite
Common in nature continental arc systems. (57-63% SiO2)
Silicic andesite
Contains 63-68% SiO2 but extended by TAS classification up to 77%.
Dacite
Generally composed of 66-69% SiO2. Also called as Shoshonite
Trachyandesite
Contains >69% SiO2 and associated with explosive silicic eruptions.
Rhyolite
Contains 68-73% SiO2 and is not typically recognized by the IUGS as an official classification for rocks in between rhyolites and dacites.
Rhyodacite
Develop on the overlying ocean lithosphere plate above the subduction zone.
Island Arcs
Contains calcium plagioclase and quartz, with minor potassium feldspar, biotite, and hornblende.
Tonalite
or plagiogranite, are granodioritic rocks with half to two- thirds sodium plagioclase for the total feldspar content.
Trondhjemite
Produce low potassium arc tholeiite basalts, as well as relatively rare rocks such as boninites and adakites.
Young island arc systems
High magnesium, intermediate volcanic rocks that contain a SiO2-saturated groundmass.
Boninite
Silica-saturated rocks with high Sr/Y and La/Yb ratios and low HFS concentrations.
Adakite
Archean associations found in archean subduction zones where adakite formation is linked to.
Tonalite, Trondhjemite and Granodiorite (TTG) Associations
Develops behind an arc during lithospheric extension occurring in the overlying plate and can erupt a diverse suite of volcanic rocks.
Backarc basins
Mature convergent arcs that involve the subduction of ocean lithosphere beneath thick continental lithosphere, where the subducting lithosphere reacts with the overlying plate, and produces more silicic magmas.
Continental margin arcs
Arcs producing voluminous granodiorite, diorite, granite, and tonalite plutons which can erupt onto the Earth’s surface and produce composite volcanoes.
Ocean-continent convergent margins
Dark-colored, trachyandesites, potassium-rich
Shoshonite
Continent-continent collisions where the lower continental lithosphere does not subduct to great depths, but essentially breaks off and underplates the overlying continental lithosphere plate, producing a doubly thick lithosphere
Continent collision zone
Fault- bounded, deformed rock sequences (F1) that mark the site of present or former convergent margins.
Alpine orogenic complexes
An intensely sheared, heterogenous rock assemblage embedded within a highly deformed mud matrix.
Tectonic melange
What are the types of Granite?
M-type Granites
I-type Granites
S-type Granites
A-type Granites
Derived from mantle-derived parental magmas
M-type granites
Generated by the melting of an igneous protolith
I-type granites
Produced by the melting of sedimentary crustal rocks in collision zones.
S-type granites
Anorogenic rocks produced by activities that do not involve the subduction and collision of lithospheric plates.
A-type granites
Magma generation and igneous rock suites generated within lithospheric plates which may be initiated by hotspot activity, continental rifts, or overthickened continental lithosphere.
Intraplate Magmatism
Greatest manifestations of intraplate magmatism on Earth, encompassing >106 km³ in volume, which occur as both oceanic flood basalts and continental flood basalts, and is generally basaltic in composition.
Large igneous provinces (LIP)
LIPs that are more silicic in composition
Silicic large igneous provinces (SLIP)
Involves Ocean islands and ocean plateaus that formed above mantle hotspots that erupt anomalously high volumes of tholeiitic and alkalic basaltic lava onto the ocean floor.
Oceanic Intraplate Magmatism
Volcanic landforms that rise upward above sea level
Ocean islands
Volcanically produced peaks below sea level.
Seamounts
Broad, flat-topped areas that result from massive outpourings of lava flowing laterally from source vents.
Oceanic Plateaus
Geochemically distinct suite of rocks that are more alkalic, less depleted, and may be somewhat enriched with incompatible elements that is interpreted to represent different mantle sources areas.
Oceanic Island Basalts (OIB)
Magmatism and volcanism within continental plates that can produce continental flood basalts continental rift assemblages, (CFB), bimodal volcanism, layered basic and ultrabasic intrusions, ultrabasic suites.
Continental Intraplate Magmatism
Huge outpourings of basalts within continental plates.
Continental Flood Basalts (CFB)
A continental feature that produce a wide array of rocks, including alkalic basalt, as well as alkalic and silicic rocks.
Continental Rifts
The widespread occurrence of basalt and rhyolite without significant andesite and occurs at continental rifts and hotspots underlying continental lithosphere.
Bimodal Volcanism
Anorogenic bodies injected into stable continental cratons at moderate depths.
Layered Basic-Ultrabasic Intrusions
Ultrabasic volcanic rocks found exclusively in Archean greenstone belts, which are associated with valuable metallic ore deposits.
Komatiite
Brecciated, magnesium-rich, ultrabasic rocks that rapidly rise to Earth’s surface via cylindrical diatremes from deep within the mantle and are intimately associated with diamonds.
Kimberlites
Shallow intrusive to volcanic rocks that contain >20% CO3 minerals, and form in stocks, dikes and cylindrical structures primarily at continental rifts.
Carbonatite
Only known active volcano to produce carbonatites.
Ol Doinyo Lengai
Magnesium-rich, volatile-rich, porphyritic rocks containing mafic phenocrysts, and are associated with kimberlites and continental rift zones.
Lamprophyre
Potassium-rich, peralkaline rocks containing pyroxene and olivine minerals enriched in barium, lanthanum, and zirconium, but relatively poor in CO2.
Lamproite
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